Cemented chromium and chromium boride material and production thereof



Sept. 24, 1957 Filed June 22, 1953 lr E- 5.

F. W. GLASER CEMENTED CHROMIUM AND CHROMIUM BORIDE MATERIAL AND PRODUCT ION THEREOF 3 Sheets-Sheet 1 IN V EN T R.. qN/e ld. 456e M, M i JW,

ff/VV Sept. 24, 1957 Filed June 22, 1953 F. W. GLA E 2,807,075 CEMENTED CHROMIUM AND CHROMIUM BORIDE MATERIAL AND PRODUCTION THEREOF 3 Sheets-Sheet 2 a 4a 6a o fao Sept. 24, 1957 F. w. @LASER 2,307,075

CEMENTED CHROMIUM AND CHROMIUM BORIDE MATERIAL AND PRODUCTION THEREOF 3 Sheets-Sheet 5 Filed June 22, 1953 .w R4 @ma M L. M mw a M a a M F f ,A Y f www# w m w Mm B 0N M FM d ...NW w, w .2 7J Nw m 55m wf fw. d Mm f im my im Ek y .p2 5 M M M/ L 0 W n T a d a u a Edf W a c. d a 4 a a I C a A J I W 6 W F w 4 L E M M a# K f N Z MM W i A M L 2 m. M y 3 z a .a a o aa 2 J. 4 H m f Y f United States Patent C CEMENTED CHROIVH'UM' AiN-D CHROMIUlV BORIDE MATERIAL AND PRODUCTION 'ITHEREOI` Frank W. Glaser, New York, N. Y., assigner to Borolite Corporation, Pittsburgh, Pa., al corporation of 'Delaware ApplicationfJune 22, 1953Serial No. 363,972 4 Claims. (Cl. 2`9-182.5)

-highresistance to corrosionr underYad-verse conditions at which other known materials have only a `short life `or are-otherwise unsatisfactory.

Chromium has long been known as a material` which `has high corrosion-resistance at hightemperatures, this corrosion-resistance being securedjby the' formation of .a ,corrosion-resistant` chromium `oxide surface stratum on `the chromium surface which` is exposed to oxidizing combustion ga-ses at high temperatures.. Howeveichromium has only relatively low creep resistance. at high temperatures, and for this reason, chromium cannot be used `byiltselfin applications which require low `creep resistance at high temperatures.

The bestprior eiortsto combine chromium with boron for its low creep resistance at elevated temperatures. are described in Cole et al. Patent 2,088,838, the material of .this patent having become known as Colmonoy. This ,Colmonoy material contains as principal ingredients three chromium borides CrB2, CrB and CrsBz, and the conglomerate of these three chromium borides, as described v in the Colmonoy patent may also containin free, compounded or alloyed state, minor further additions of chromium, boron, aluminum and iron.

However, t-he Colmony material `of the type described in4 Cole et al'. Patent 2,088,838, has excessive brittleness,

and could not be utilized for producing, either by itself orY by cementing particles thereof with any of the' known cementing additions, -a cemented material that would exhibit the required high strength, and that `would have the capacity of elastically yielding under a load at elevated temperatures as required in certain. gasturbine parts.

One phase of the present invention is based `on the discovery that by combining chromium with dichromium boride CraB, which has many desirable characteristics that make it superior to other boron rich chromiumboride compounds, to wit, CrBz, CrB and CraBz, there is obtained a material of high hot strength and corrosionresistance which is much superior to either CraB oriCr when used alone for producing gas turbine buckets, rotors and. other high temperature metal. parts which `present critical corrosion problems. An X-ray study of the crystalline structure of dichromium boride CrzB indicates that it has an orth'orhombic cell of the following parameters:

Its specific gravity is 6.2y gram/cc., and it has a micro hardness of Vickers. DPH 11433.. lt has a melting point between about 11650. and' 17`50 C. The dichromium bolide CrzB will take about 20% chromium in solid solu- Patented Sept.` 24, 1957 tion. It forms a metal rich phase of'a muchy higher order of ductilitythan other refractory metal borides.

One phase `of the present invention relates to a new structural material which in addition to exhibiting high strength, heat shock resistance and corrosion resistance at elevated temperature, also exhibits a substantial desired degree of ductility, the new material combining the dichromium .boricle` CrzB with chromium. BychoosingI the proportion yof the excess chromium, the newdichromium boride-excess-chromium material may be given the desired degree of strengthv and corrosion resistance aswell as the desired degree of ductility at elevated temperatures, depending on the temperature .at'which it is to be used.

Thenew dichromium horideaexcess-chromium material ot" the invention may to advantage also contain a pro-l portion `of chromium oxide CrzOa. For best. results,` it should be free of. carbon impurities greater than about 011%,4 and of iron `impurities greater than. 0.1 to 0.15%. (Throughout the speciiicati-on and claims, all proportions are-giveni-n weight,` unless otherwise specically'stated.) This new material ofthe invention may be described as boron-poor or boron-deficient` dichromium boride `material. For securing the minimum required increase. in creep-resistance over pure chromium, it should `contain a minimum 'of about 0.7% boron, and its boron content should not be higherthan about 10%. Tests indicate that theC-rzBl plus Cr material has a eutecticl composition Vfor 4.8% boron content, the eutectic temperature being .about 15 00 C. at which the materialV forms a li'quid phase.

The dichromium boride material CrzB without and with the excess of chromium, may be produced by direct synthesis of chromium and boron having a purity of atleast about 957%,` but for best results the impurities should not exceedv about 2.5%, and preferably said impurities should be only about 0.5% or less; The commercially available electrolyticV chromium and commercially'available amorphous boron of such purity may be usedy for producing this material. Instead ofamorphous boron, crystalline boron may be used. In producing the dichromium boride with the desired excess of chromium ice `or without it,v chromium and boron powders are mixed. in

proportions corresponding to the stoichiometric proportions of CrBz with or without the desired excess of chromium, and the mixed powder ingredients are subjected `to a heat treatment in which the small amount of boron powder present combines with the chromium into dichromium boride CrzB.

Satisfactory results are obtained with mixing the required proportions of the chromium powderand the boron powder which have been comminuted to a particle size of 4 t-o 5 microns, and thereafter subjecting the powder mixture to a further mixing treatment, as in a ball mill, to effect thorough mixture of the. different powders and further comminution of the particle size to. :about 1/2. to `2 microns. Good results are obtained by comminuting the individual powders in a gas vortex type pulverizing mill to a particle size of 4 to `5 microns and. subsequently ball-milling the mixture of the two powders for about 54 hours. Tests indicate that prolongation of the mixing by ball-milling beyond about 54 hours, does not result in an improved final material.

The ball-milled mixture of the properly proportioned powder ingredients is then heated in a protective atmosphere within a crucible at a temperature of 1300 to 135.0 C., or in general between 1200-1500" C. until the amorphous boron has been purified and the reaction between the chromium and boron has reached equilibrium condition. Good results are obtained with a heat treatment of from one to two hours which yields the body of dichromium boride CrzB with or without the desired ex.- cess of chromium, depending on the proportions ofV the powder ingredients in the starting powder mixture. The

amorphous boronrcontains magnesium oxide as a maior impurity ingredient, and the heat treatment at 1300" C.

2,807,075 Y u g In 'an'alternative procedure, `the dichromium boride ',CrzB'WithvorV without the desired excess of chromium .may be obtained by mixing'electrolytic chromium and `purified boron in the desired final proportions, in which case the initial comminution of the `individual powders by .a gaseous vortex pulverizing mill is not required. This procedure requires boron of the highest possible purity,

' of not less than about 95% of vits total as chemically analyzed. The properly proportioned mixture of the electrolytic chromium and pure` boron powder ingredients is then ball-milledto mix, and then subjected to a similar heat treatment'in a graphitecrucible within a hydrogen atmosphere at,1300 C. to 1350 C. until the reaction .between the chromium and the boron present in the mixture yields at equilibrium-condition the dichromium boride CrzB, with or without the desired excess of chromium.

By keeping the boron content of the powder mixture Vtovatrnost the resulting material will be free of .the boron richer chromium borides, to-wit, CrB, CrBz Sand CraBz.V f Y o VTheV carbon impurities of the powder mixturev used in making the material of the invention should be kept not larger than 0.1%. Iron impurities of about 0.1 to 0.15% Vthat' are'introduced by ball-milling with steel balls, are not harmful, but may beV eliminated by leaching with HC1. I f 2^ Shaped articles of high strength, creep resistance, heat- Vshock resistance', and resistance to corrosion, at elevated` temperatures, and'having also desired ductility at such i temperatures, may be produced vout of powder particles of dichromium boride CrzB and excess. chromium, by -powder'metallurgy techniques or ceramic techniques, including hot-pressing as well as cold-pressing or hydrof'static pressing of the desired shapes, followed'by sintering. The hot-pressing andthe sintering of compacted bodies should be carried out at temperatures between about 1350 and 1500 C. Where the boron content of lthe `combined body is in excess of 4%, the heat treatlment and sintering temperature may be increased up to 1700 C. Good results are obtained by hot-pressing and sintering'of the compacted powder bodies in an oxid- 'izing atmosphere, such as air, or in an inert atmosphere such as helium or argon or in vacuum. Nitrogen atmospheres should not be used in producing cemented bodies ofthe invention. It is also detrimental to carry on the ,hot-pressing or sintering treatments of cemented bodies of the invention in a carbonaceous and/or hydrogen atmosphere, since carbon and hydrogen will have a tendency to reduce chromium oxides which are desirable in the final product. If a carbonaceous and/or hydrogen latmosphereis employed, embrittlement and lowering of the physical properties of the resulting cemented bodies will take place.

In general, satisfactory cemented bodies may be produced by hot-pressing at temperatures of 1400 to 1900 C. or, in general, between 1000 and 2000 C. with pressures of 1/2 to 11/2 t. s. i. (tons per square inch). When making bodies by compactingand sintering procedures, good results are obtained by compacting thel powder mixture with'a pressure of 2A to 4 t. s. i. and subsequently sintering the compact at temperatures above about 1350" C. and close. to the melting point of chromium metal. According to available data, pure chromium 4.melts atV about 1800 C. m20"l C. l v When producing cemented bodies by hot-pressing, it is desirable to make the hot-pressing die of a material which does not produce within the die-cavity-a cari bonaceous atmosphere. l Dies ofY ,zirconium diboride ZrBz bonded with 2 to 7% excess boron containing 4 to 33 atomic percent carbon in solid solution or of silicon carbide bonded by silicon nitride, are suitable, in which case the hot-pressing may be Ydone with pressures up to about 10 to 12 t. s. i. If a graphite die is used, the die cavity should be coated with a refractory cement such as zirconium oxide cement, titanium oxide cement, or like cements which are free of or very poor in carbon at the hot-pressing temperature.

The physical characteristics of cemented bodies of the invention will now be described by a series of curve Figs. 1-7 of the drawings are graphs showing the per-V formance of the various compositions of CrzB and chromium. f Y Y l Fig. l is a curve diagram whereinrgraph 11 indicates the density Vof a cemented-body formed ofVfCrzB plus Cr for different proportions of CrzB as it increasesfrom zero to 100%. Y It will benotedl that the increase ofthe Cr2B content decreases the density of the material.

VFig. 2 is a curve diagramfwherein graph'lines 12, 13

vshow the transverse rupture strength for cemented material CraB-i-Cr at 1000 C. for different contents of Cr2B as it increases from zero to 100%. This graph shows that the transverse rupture strength is at maximum for a body containing about 35% dichromium boride CrzB.

Difliculties lhave `been encountered with bodies having a Cr2B content of Vmore than 50%k becausey such bodies have considerably lower ductility than bodies with a smaller CrzB content. o l

Fig. 3 is a curve diagram wherein graph 5 shows the average Rockwell A hardness for bodies Vformed of CmB plus Cr, for different proportions of'CrzB Yas it increases.

from zero to 100%. It shows that maximum hardness is obtained with abody containing about CraB. j

Fig. 4 is a curve diagram wherein graph 16 shows the electrical resistivity of a cemented body formed of Cr2B-|Cr, for different proportions of CrzB as it increases from zero to As shown by the graph, the electrical resistivity increases linearly for bodies wherein the CrzB content increases above 10%, but there is a change in the slope of the graph between zero and 10% increase of CrzB. A study has shown that chromium i Will take approximately .9 to 1% byV weight of boron in solutionand this fact may explain the 'break in this graph at 10% CrzB'content, representing a composition containing about .95% boron. v k

Fig. 5 shows graphs of the oxidation resistance in air and in combustion atmospheres of three bodies of different CrzB content as a function of time.V Graph 17-1 indicates the oxidation resistance fora body containing 15% CrzB, graph 17-2 for a body containing 30% CrzB, and graph 17-3, for a body containing 40% CrzB, the balance chromium. The graphs of Fig. 5 show that the corrosion resistance of these materials is excellent and that after the rst 200 hours, in which a chromium oxide iilm develops on the exterior of the body, the further Weight gainv is very small. Even after 1000 hours exposure to oxidizing conditions, the appearance of all body specimens subjected to the corrosion tests was excellent and they kept their sharp corners and their original transverse rupture strength. n i

' Fig. 6 is a curve diagram wherein curve 18 shows `the transverse rupture strength for a body containing 30% CrzB and 70% Cr, as a function oftemperature. Curve 18 shows that thematerial hadV a transverse rupture strength inexcess of 90,000 p. s. i. (pounds per square inch) in the temperature range between 1200 and Fig. 7 is a curve diagram wherein graph 19 shows the deflection rate under a transverse load of 40,000 p. s. i. (pounds per square inch) at 1000 C. for bodies of CrzB-I-Cr, as the CrzB content is increased from zero to about 80%. The best grade of titanium carbide deflected about 4 l05 inches per minute under similar test conditions, corresponding to the performance of the material of the invention containing Cr2B/50% Cr.

As explained above, cemented bodies of the invention containing CraB-l-Cr, may be produced either by hotpressing or by compacting and sintering. To obtain materials of high density approximating the theoretical densities, hot-pressing at l500 C., at which a liquid phase is formed, with subsequent sintering at the same temperature for 30 to 60 minutes, is desirable. With hot-pressed material which had about 90% theoretical density, almost its full density could be achieved by additional sintering of one hour in air at about 1500 C. Further sintering beyond one hour did not appear to affect the physical properties of the material.

According to a further phase of the invention, a desired material of high corrosion resistance may also be obtained by combining the compound CrzB in powder form with silicon. Thus, whereas Cr2B by itself, when subjected to oxidation at 1000 C. in air, exhibited after two hours a weight gain of 15 to 25 milligrams per cm?, an addition of 5 to 20% of silicon reduced the weight gain under the same conditions to 0.3 milligram per cm?. The addition of 5 to 20% silicon metal to CrzB may be accomplished either by mechanical mixing of the ingredients or a synthesis of the component elements of the ingredients in the desired proportions and producing cemented bodies in ways similar to those described above in connection with -the production of CrzB-l-Cr. Cemented bodies consisting of CrzB with 5 to 20% silicony have a transverse rupture strength of 75,000 to 90,000 p. s. i. at 1000" C., and about 56,000 to 60,000 p. s. i. at room temperature. Satisfactory material may be made with as much as 50 to 60% silicon.

By way` of example, to produce the desired CrzB-l-Si material, powders of the CrzB-or of Cr and B in proportions corresponding to the desired amount of CrzB-are mixed with the desired proportion of silicon metal of as high purity as commercially available, which has been comminuted to an average particle size of about 2 to 3 microns. The powder mixture is then further mixed by ball-milling until they have been thoroughly mixed. Good results are obtained by mixing through ball-milling for about hours. The powder mixture is then hot-pressed, followed by sintering or cold-pressing followed by sintering, in the manner described above for CrzB-i-Cr, to produce cemented bodies of CrzB-l-S.

By way of example, a body produced by hot-pressing a body containing CrzB and 20% silicon, had the following physical characteristics:

Density 5.1 gram per cc. Electrical resistivity 60 microhm cm. Transverse rupture strengthat 1000o C Above 100,000 p. s. i.

This material has in general, properties similar to the material of the invention described above which contained 50% Cr2B-I-50% Cr.

It will be apparent to those skilled in the art that the novel principles of the invention disclosed herein in connection with specic exemplications thereof will suggest various other modications and applications of the same. It is accordingly desired that the present invention shall not be limited to the specific exemplications shown or described therein.

l claim:

1. A hard body of high strength consisting of cemented particles containing essentially 15% to 45% of CrzB and 85% to 55% of a substance selected from Cr and Si, said body being substantially free of CrB, CrBz and Cr3B2.

2. A hard body of high strength consisting of cemented particles containing essentially 15% to 45% of CrzB and 85% to 55% of Cr, said body being substantially free of CrB, CrBz and CraBz.

3. A hard body of high strength consisting of cemented particles containing essentially 40% to 95% Cr2B and 5% to 60% Si, said body being substantially free of CrB, CrBz and CraBz.

4. A hard body of high strength consisting of cemented particles containing essentially 95% to 80% CrzB and 5% to 20% Si, said body being substantially free of CrB, CrBz and CrsBz.

References Cited in the le of this patent UNITED STATES PATENTS Laise June 22, 1937 Boecker Aug. 29, 1939 OTHER REFERENCES 

1. A HARD BODY OF HIGH STRENGTH CONSISTING OF CEMENTED PARTICLES CONTAINING ESSENTIALLY 15% TO 45% OF CR2B AND 85% TO 55% OF A SUBSTANCE SELECTED FROM CR AND SI, SAID BODY BEING SUBSTANTIALLY FREE OF CRB, CRB2 AND CR3B2. 